The present invention relates to a method and an apparatus for correcting aberration occurring in recording/reproducing information by using reflected light of an irradiating converged light beam.
In accordance with development of Internet and spread of digital cameras and digital videos, more and more image data are processed by using computers, and hence, there is a demand for a storage with a large capacity. Optical disks are most suitably used for this purpose because of their large capacities, compatibility and ability of random accesses, and various examinations have been made on a digital video disk (DVD) and the like for attaining a larger capacity.
In general, it is necessary to minimize a focal spot in order to increase a capacity of an optical disk. Since a spot diameter is in proportion to λ/NA (wherein λ is a wavelength and NA is numerical aperture), the spot diameter can be decreased by increasing the NA. However, when the NA is larger, aberration derived from a tilt or aberration derived from an error in the thickness of a substrate is disadvantageously increased.
For example, in a DVD using an objective lens with high NA of approximately 0.6, a laminated substrate with a thickness of 0.6 mm is used in order to decrease the aberration derived from a tilt of the substrate. Also, in an optical disk of the next generation adopting a magnetic field modulation method, a single substrate with a thickness of 0.6 mm is similarly used. The thickness of the substrate is preferably further decreased in order to further increase the NA, but the tilt can be increased in a thinner single substrate because the thin substrate can be warped. In an optical disk using a substrate with a thickness of 0.1 mm or less, a reinforcing substrate is required to be adhered onto a surface other than the surface a magnetic film is adhered to. Thus, the tilt can be decreased, but a distance between the magnetic film and an optical head is increased by the thickness of the reinforcing substrate, and hence, it is difficult to adopt the magnetic field modulation method for a recording operation. Also, a substrate with a thickness of 0.1 mm or less is difficult to make compatible with a conventional recording/reproducing apparatus because the thickness is different from that of a conventional substrate.
In this manner, when the NA is large, an effect, in particular, aberration occurring in a recording/reproducing operation derived from a tilt, a difference in the thickness of respective optical disks and thickness variation in a direction along tracks, is increased. As a result, a light beam cannot be completely converged.
A tilt of an optical disk is corrected by detecting the tilt in a recording/reproducing operation of an optical disk and controlling, for example, an inclination of an objective lens in accordance with the detected tilt. In this correction, a tilt is detected by using reflected light of light irradiating the optical disk on a portion not relation with recording/reproducing information then, so that a relative tilt between a recording/reproducing laser beam and the optical disk can be corrected in accordance with the detected tilt. When the tilt is thus corrected, information can be accurately recorded/reproduced. However, since there is a distance on the optical disk between a portion for detecting the tilt and a portion for recording/reproducing information, a tilt in the portion for recording/reproducing information cannot be accurately detected. Furthermore, the correction accuracy is largely affected by an assembling error of an optical system for detecting a tilt and an optical system for recording/reproducing information.
Moreover, a tilt of an optical recording medium along a crosswise direction of tracks thereon (hereinafter referred to as a radial tilt) is comparatively easily detected. However, a tilt in a direction crossing the crosswise direction of the tracks, for example, a tilt along a tangential direction of the tracks (hereinafter referred to as a tangential tilt) is mostly constituted by an A/C component having one rotation of the disk as one cycle, and cannot be accurately corrected by the aforementioned correction method because the detecting point is different from the correcting point. Furthermore, since a response speed is low, high speed control cannot be realized. Also, since a tangential tilt is so small as compared with a radial tilt that it has been conventionally not necessary to correct the tangential tilt. However, in accordance with increase in a memory density of an optical recording medium, the correction of a tangential tilt has become more and more significant.
Japanese Patent Application Laid-Open No. 6-162541 (1994) discloses an apparatus for correcting a tangential tilt. This apparatus is a recording/reproducing apparatus for an optical disk, in which light reflected by a flat area on an optical disk is received by a photodetector so as to detect a tilt on the basis of brightness in each divided area included in the photodetector. An objective lens for converging a laser beam on the optical disk is provided with actuators on its four sides for focus control, tracking control and tilt control. These four actuators are independently driven in accordance with a detected tilt, so that a radial tilt and a tangential tilt can be corrected. In this apparatus, a tangential tilt can be accurately corrected through high speed control. However, since a tilt is detected on the flat area of the optical disk, the tilt correction cannot be performed at the same time as a recording/reproducing operation of information.